A dispenser is a device for discharging a predetermined small amount of a liquid material from a syringe that is filled with the liquid material. There is known, for example, an air type discharge device in which pressurized air is supplied as an air pulse to press a liquid material with the air pulse, and a predetermined small amount of the liquid material is discharged from a needle mounted to a tip of a syringe. When the liquid material is discharged by employing such a device, a liquid surface level within the syringe lowers each time the liquid material is discharged. However, when the liquid material has medium or high viscosity, a problem arises in that an amount of the liquid material adhering to a wall surface of the syringe increases and the liquid surface level lowers particularly in a central region of the syringe.
In order to solve the above-mentioned problem, there is proposed a technique of arranging a plunger that has flanges formed in two upper and lower stages around a peripheral cylindrical body equipped with a bottom, and substantially uniformly pressing an entire surface of a liquid material by the plunger, while preventing adhesion of the liquid material to a wall surface of a syringe (see FIG. 4(c) of Patent Document 1).
However, when the liquid material is discharged by supplying pressurized air to the plunger having the above-mentioned structure, the following problem arises. As soon as the supply of the pressurized air to the plunger is stopped, a force acting to push back the plunger is generated due to compression reaction of the liquid material that has been subjected to a compression force so far, and air is sucked into the lower side of the flange, which is positioned on the upper side, from above the upper flange. Once the air is sucked in such a manner in the normal discharge step, there is a tendency that the sucked air can no longer escape to the upper side of the relevant flange, and an amount of the sucked air rather increases with repetition of the discharge step. Because pressure of the pressurized air supplied in the pulse form to the plunger is transmitted to the liquid material through the sucked air that undergoes compressive deformation, a transmission speed of the pressure to the liquid material is reduced with the increase in the amount of the sucked air, and the amount of the liquid material discharged from the needle is changed eventually.
Aiming to solve the above-mentioned problem, the inventor has proposed, in Patent Document 1, a plunger including a tapered tip portion of which rear end has an outer diameter slightly smaller than an inner diameter of a syringe, a small diameter body portion that is formed to extend rearwards in continuation with the tapered tip portion, and that has a smaller diameter than the rear end of the tapered tip portion, a tubular portion that is formed to extend rearwards in continuation with the small diameter body portion, and that has a maximum outer diameter larger than the inner diameter of the syringe, and a drain groove formed to extend in an axial direction of the syringe or a small diameter drain bore formed to extend from the small diameter body portion up to an inner side of the tubular portion.